Succession and secession are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It suggests what word you may want to save time, but quite often, its suggestions couldn’t be more off base. Autocorrect’s suggestions are more humorous than usable.
Succession is a noun. It means who or what comes next, and in what order. In a monarchy, it refers to what prince or princesses will follow the king or queen. In England, Prince Charles in next after Elizabeth II, and his eldest son, Prince William, will follow him.
Secession is a noun. It means the act of seceding, or when a few states or provinces break away from the rest of the nation.
The following story uses both words correctly:
Suchi was surprised at the results of the latest poll. It said half the states in her country threatened secession if an extreme conservative was selected next November for succession to the presidency.